# std::replace_copy, std::replace_copy_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt, class T > OutputIt replace_copy( InputIt first, InputIt last, OutputIt d_first, const T& old_value, const T& new_value ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T > ForwardIt2 replace_copy ( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, const T& old_value, const T& new_value ); | (2) | (since C++17)
| (3) |
template< class InputIt, class OutputIt, class UnaryPred, class T > OutputIt replace_copy_if ( InputIt first, InputIt last, OutputIt d_first, UnaryPred p, const T& new_value ); |  | (constexpr since C++20) (until C++26)
template< class InputIt, class OutputIt, class UnaryPred, class T = typename std::iterator_traits <OutputIt>::value_type > constexpr OutputIt replace_copy_if ( InputIt first, InputIt last, OutputIt d_first, UnaryPred p, const T& new_value ); |  | (since C++26)
| (4) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred, class T > ForwardIt2 replace_copy_if ( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred p, const T& new_value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred, class T = typename std::iterator_traits <ForwardIt2>::value_type > ForwardIt2 replace_copy_if ( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred p, const T& new_value ); |  | (since C++26)

Copies the elements from the range `[`first`,` last`)` to another range beginning at d_first, while replacing all elements satisfying specific criteria with new_value.

1) Replaces all elements that are equal to old_value (using operator==).
3) Replaces all elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If any of the results of the expressions *first and new_value is not writable to d_first, the program is ill-formed.

If the source and destination ranges overlap, the behavior is undefined.

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range
old_value | - | the value of elements to replace
policy | - | the execution policy to use
p | - | unary predicate which returns ​true if the element value should be replaced. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
new_value | - | the value to use as replacement
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.

### Return value

Iterator to the element past the last element copied.

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons using operator==.
3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

replace_copy
    template<class InputIt, class OutputIt, class T>
    OutputIt replace_copy(InputIt first, InputIt last, OutputIt d_first,
                          const T& old_value, const T& new_value)
    {
        for (; first != last; ++first)
            *d_first++ = (*first == old_value) ? new_value : *first;
        return d_first;
    }
replace_copy_if
    template<class InputIt, class OutputIt, class UnaryPred,
             class T = typename std::iterator_traits<ForwardIt>::value_type>
    OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first,
                             UnaryPred p, const T& new_value)
    {
        for (; first != last; ++first)
            *d_first++ = p(*first) ? new_value : *first;
        return d_first;
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (3,4)

### Example

Run this code

    #include <algorithm>
    #include <complex>
    #include <iostream>
    #include <vector>

    void println(const auto& seq)
    {
        for (const auto& e : seq)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::vector<short> src{3, 1, 4, 1, 5, 9, 2, 6, 5};
        println(src);
        std::vector<int> dst(src.size());
        std::replace_copy_if(src.cbegin(), src.cend(),
                             dst.begin(),
                             [](short n){ return n > 5; }, 0);
        println(dst);

        std::vector<std::complex<double>> src2{{1, 3}, {2, 4}, {3, 5}},
                                          dst2(src2.size());
        println(src2);
        #ifdef __cpp_lib_algorithm_default_value_type
            std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
                [](std::complex<double> z){ return std::abs(z) < 5; },
                {4, 2}); // Possible, since the T is deduced.
        #else
            std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
                [](std::complex<double> z){ return std::abs(z) < 5; },
                std::complex<double>{4, 2});
        #endif
        println(dst2);
    }

Output:

    3 1 4 1 5 9 2 6 5
    3 1 4 1 5 0 2 0 5
    (1,3) (2,4) (3,5)
    (4,2) (4,2) (3,5)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be CopyAssignable (and EqualityComparable for `replace_copy`), but the value type of `InputIt` is not always `T` | removed the requirement
LWG 337 | C++98 | `replace_copy_if` only required `InputIt` to meet the requirements of LegacyIterator[1] | corrected to LegacyInputIterator

1. ↑ The actual defect in the C++ standard is that the template parameter `InputIterator` was misspecified as `Iterator`. This affects the type requirements because the C++ standard states that for the function templates in the algorithms library, the template type parameters whose name ends with `Iterator` imply the type requirements of the corresponding iterator categories.

### See also

replacereplace_if | replaces all values satisfying specific criteria with another value (function template)
removeremove_if | removes elements satisfying specific criteria (function template)
ranges::replace_copyranges::replace_copy_if (C++20)(C++20) | copies a range, replacing elements satisfying specific criteria with another value (algorithm function object)

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